Subscriber modems are used for transmitting data between a subscriber terminal which can be connected to the subscriber modem, and a switching office. For this purpose, the subscriber modem is connected via a subscriber line to an associated switch modem within the switching office. The various switch modems within the switching office are connected to a switching device, particularly to a multiplexer circuit, in order to switch the various subscriber lines through to a broadband data transmission line. Between the subscriber modem and the associated switch modem, the data are transmitted bidirectionally via various transmission frequency bands. In this arrangement, the data from the subscriber modem to the associated switch modem are transmitted via a first transmission frequency band and the data from the switch modem to the associated subscriber modem are transmitted via a second transmission frequency band. The subscriber modem and the switch modem are so-called FDD (Frequency Division Duplex) modems. The modems are for use in public and private networks.
On the subscriber lines, the data are transmitted by means of different line coding methods. In these methods, a distinction is made between so-called multi carrier methods and single-carrier methods. In the single-carrier methods, a carrier signal having one carrier frequency is used for signal modulation. The single-carrier methods or else SCM (Single Carrier Modulation) methods comprise, for example, the QAM or CAP methods. An example of a multi-carrier method is the DMT (Discrete Multi-Tone) method used in ADSL modems.
Both voice signals and data signals are transmitted via the subscriber lines. The voice and data signals are transmitted in different frequency bands. The data are transmitted via a so-called downstream and upstream channel which in each case use different transmission frequency bands. The frequency band used in VDSL (very high bit rate digital subscriber line) systems is between 138 kHz and 30 MHz. In the frequency band allocation for the data transmission, in each case one transmission frequency band is used for the downstream channel and the upstream channel in the simplest case, that is to say a total of two transmission frequency bands. It is also possible to use more than one transmission frequency band for both directions of transmission. Single-carrier methods use only one signal carrier per transmission frequency band whereas multi-carrier methods use a number of carriers per transmission frequency band.
The subscriber lines of the various subscriber modems are bundled at the switching office to form a cable bundle consisting of a number of data transmission lines. As a rule, the subscriber lines used for the data transmission are conventional two-wire telephone lines which are unshielded. For this reason, an interfering signal cross talk may occur between the subscriber lines.
In the cross talk, a distinction is made between so-called far end cross talk and near end cross talk. In the case of the near end cross talk (NEXT), for example, an unwanted injection of the signal transmitted by a first switch modem into the access line of another switch modem occurs. In the case of far end cross talk (FEXT), the signal transmitted from a subscriber modem to the associated switch modem is injected into another subscriber line in the area of the cable bundle and thus interferes with the received signal of another switch modem.
FIG. 1a shows a conventional subscriber access network with a switching office containing a number of switch modems VM to which subscriber modems TM are in each case connected via access lines AL. In the example shown in FIG. 1, two subscriber modems TM are connected to the switching office. The first subscriber modem TMN is located in the local area of the switching office, for example only 300 m away from the switching office. The other subscriber modem TME is remote from the switching office, for example at a distance of over 1000 m. The attenuation of the data transmission signal sent by a subscriber modem depends both on the transmit signal frequency and on the distance between the switch modem and the subscriber modem. If the remote subscriber modem TME and the local subscriber modem TMN transmit the transmit signal at the same power via the upstream channel, a far end cross talk will occur in the area of the switching office from the subscriber line ALN of the local subscriber modem TMN onto the subscriber line AE of the remote subscriber modem TME. In this case, the transmission of the remote TM is disturbed more than in the case in which a number of modems transmit from the same distance. The same situation occurs if there are a number of switching offices at different distances from the subscriber modems (FIG. 1b). In the last-mentioned case, the downstream frequency band is also subsequently affected by the method according to the invention.
FIG. 2 shows the variation of the spectral power density PSD in the subscriber access network shown in FIG. 1. The transmit signal of the remote subscriber modem TME exhibits a constant transmit power spectrum at the location of the remote subscriber modem TME. In the area of the local subscriber modem TMN, the transmission spectrum of the remote modem TME is already attenuated and is below the transmit power spectrum of the local subscriber modem TMN. At the switching office, both transmit signals are attenuated due to the line attenuation, although the spectral power level of the local subscriber modem TMN, received by the switching office, is clearly higher than the spectral power level of the remote subscriber modem TME. For this reason, far end cross talk (FEXT) occurs.
In conventional systems, therefore, a transmit power reduction as shown in FIG. 2b is performed at the local subscriber modem TMN. This transmit power reduction is also called power back-off PBO. For this purpose, the transmit power of the local subscriber modem TMN is constantly lowered over the entire transmission frequency band. As a result, the power difference between the received signals on the various subscriber lines AL received signals is reduced in the area of the switching office and far end cross talk TEXT is thus reduced. As can be seen in FIG. 2b, the difference between the attenuated data signal received from the local subscriber modem TMN and the received signal received from the remote subscriber modem TME is less than in the case without transmit power reduction shown in FIG. 2a in the area of the switching office.
It is mainly in the case of mixed operation of modems with different methods for transmit power reduction that there are differences between the various transmit signal powers leading to an inadmissible far end cross talk.